Extrusion method, extruder and extruded product

ABSTRACT

An extruder according to the invention comprises at least two annular conical feed gaps ( 9 ) one within the other, formed between a rotatable rotor ( 1 ) and a stator (4, 5). At least some of the material to be extruded is subjected in the different feed gaps ( 9 ) of the extruder alternately to a higher pressure and then to a lower pressure. The rotor ( 1 ) or stator ( 4, 5 ) between the feed gaps ( 9 ) can then be balanced such that the pressure effect provides a hydrodynamic bearing, whereby, when the extruder is used, even at worst a very small force is exerted on die other bearings of the rotor ( 1 ) or stator ( 4, 5 ). By the method of the invention, a product can also be produced which contains cross-linked and at least partly oriented polyethylene.

FIELD OF THE INVENTION

The invention relates to an extrusion method in which material isextruded by an extruder comprising at least two annular conical feedgaps one within the other, provided between a rotatable rotor and astator, in the method, extrudable material is extruded from the extruderby means of primarily helical curve-shaped grooves located in the rotorand/or the stator between the rotor and the stator and by means offlights located between the grooves.

The invention also relates to an extruder that comprises at least twoannular conical feed gaps one within the other, provided between arotatable rotor and a stator; the rotor and/or stator comprising atleast primarily helical curve-shaped grooves and flights between them toextrude the extrudable material from the extruder.

BACKGROUND OF INVENTION

The invention further relates to a product extruded with the method.

U.S. publication 3,314,108 discloses an extruder comprising two conicalstators and a conical rotor rotatably arranged between the stators. Theplastic material to be extruded is supplied via two different conduitsto different sides of the rotor. The rotor is provided with helicalgrooves by means of which the material is transferred towards an orificeat the narrower end of the extruder. The flows of material on differentsides of the rotor exert a force on the rotor, whereby the bearings ofthe rotor must be very firm. Further, the clearances between the rotorand the stator should be adjusted carefully so as to prevent a back flowof the extrudable material back toward the material inlet. A smallclearance is naturally a difficult structure to implement, and excessfriction is easily generated between the rotor and the stators. On theother hand, a bigger clearance would add to the back flow and therebyreduce extruder output.

EP 0,422,042 discloses an extruder with a plurality of conical statorsand conical rotors rotatably arranged between them. The extrudablematerial is fed via one conduit to the forward end of each rotor, therotor comprising, at the inlet, holes through which the material canalso flow to the other side of the rotor. The material is transported tothe end of the extruder by means of grooves arranged on both sides ofthe rotor. The different sides of the rotor are also subject to greatforces in this device, requiring firm bearings. A further problem is aback flow over the flights between the grooves back toward the materialinlet and an accurate arrangement of clearances between the rotors andstators.

U.S. Pat. No. 3,689,181 discloses an extruder comprising a plate-likerotor and a plate-like stator. Recesses and protrusions are arranged inthe rotor and the stator so as to improve the mixing of the material.The material is extruded from the extruder mainly by centripetal force,making the extruder quite ineffective. Furthermore the structure of theextruder is such that it requires very firm bearings.

GB 2,202,783 describes an extruder with a plurality of partially conicalrotors on top of each other. The rotors are provided with grooves thattransport the extrudable material out of the extruder. The grooves maybe provided with mixing means, e.g. rods, to reduce the cross-sectionalarea of the groove. These means mix the material, but they significantlyhamper material flow and thus reduce extruder output.

SUMMARY OF INVENTION

It is an object of this invention to provide an extrusion method and anextruder in which the above drawbacks can be avoided. It is a furtherobject of the invention to provide an extruded product with goodcharacteristics.

The extrusion method of the invention is characterized in that at leastpart of the extrudable material is subjected alternately to higher andlower pressure in the feed gaps of the extruder, the flow rate of thematerial alternately increasing and decreasing, and that at least partof the extrudable material is led from one groove to another.

The extruder of the invention is further characterized in that theextruder is arranged to subject at least part of the extrudable materialalternately to higher and lower pressure and to lead at least part ofthe extrudable material from one groove to another.

The extruded product of the invention is further characterized in thatthe product contains orienting material, such as liquid crystal polymer.

It is an essential idea of the invention that the extruder comprises atleast two annular conical feed gaps one within the other, and that atleast part of the extrudable material is subjected alternately to ahigher and then to a lower pressure in the different feed gaps of theextruder. The idea of a preferred embodiment that in order to providethe desired pressure effect, at least part of the extrudable material isled alternately to a lower and then to a higher space. It is the idea ofanother preferred embodiment that the flights between the grooves thatsupply the extrudable material are bevelled so that the extrudablematerial can flow in the circumferential direction of the device, butowing to the pressure effect, axial back flow can be prevented. The ideaof a third embodiment that the groove supplying material is arranged tobecome shallower and deeper in the flow direction of the material, andat the end of the shallowing portion the rotor or stator between thefeed gaps comprises an hole via which at least part of the material canflow to the other side of the rotor or stator. If desired, the holes canbe formed conical, i.e. they can be arranged to converge.

It is an advantage of the invention that by a varying pressure effect ondifferent sides of the rotor or stator the rotor or the stator enablesbalancing the rotor or stator between two different feed gaps so thatthe pressure effect provides a hydrodynamic bearing, the other bearingsof the rotor or the stator being subject to little or no force at allduring operation of the extruder, whereby the structure of the otherbearings can be lighter since they do not need to endure great forcesduring their whole service life. A further advantage is that theextrudable material can be efficiently mixed. When the material is ledalternately to a lower and a higher space, the extrudable material canbe heated further by friction heat generated by shear forces. At thesame time heat transmission and consequently temperature equalizationimproves as the materials pass via a lower space. Furthermore, as theproduct is subjected to a higher pressure, a product can be produced inwhich orientation can be generated as the flow cross-sectional surfaceconverges. By arranging holes in the rotor or the stator through whichthe material supplied to the lower point can flow from one feed gap toanother, the possible pressure difference in different parts of therotor or stator can be evened out, which further reduces the forceexerted on the rotors or the stators. When the holes are arranged toconverge, the material passing through can be oriented in the convergingthrough-holes. When the flights between the grooves that transport thematerial are bevelled, in the flight will be formed a pressure function,by the action of which the axial back flow of the equipment can bereduced. The clearance between the rotor and the stator can, however,also be made bigger without excessive increase in the back flow, thefriction between the rotor and the stator being reduced to a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the attached drawings,in which

FIG. 1 is a schematic side view of the rotor of an extruder according tothe invention,

FIG. 2 is a cross section of a part of the rotor according to FIG. 1,

FIG. 3 schematically shows the pressure functions of a flight betweenthe grooves of a rotor according to FIG. 1,

FIG. 4 is a schematic cross-sectional side view of another extruderaccording to the invention,

FIG. 5 shows the device according to FIG. 4 in cross section in thedirection of the rotor grooves,

FIG. 6 is a schematic side view of a rotor part of a third extruderaccording to the invention,

FIG. 7 is a cross section of a part of the rotor of FIG. 6,

FIG. 8 is a schematic side view of the rotor of a fourth deviceaccording to the invention, and

FIG. 9 is a schematic cross-sectional view of a detail of the rotoraccording to FIG. 8 in the axial direction.

DETAILED DESCRIPTION

FIG. 1 is a schematic side view of a rotor 1. The rotor 1 compriseshelical curve-shaped grooves 2, by the action of which the extrudablematerial moves upward in the figure while the rotor rotates at acircumferential speed v. The extrudable material is supplied to thewider end of the rotor 1 in a manner known per se. For the sake ofclarity, the supply means and the rotor 1 rotation means are not shownin the figure. Flights 3 are arranged between the helical curve-shapedgrooves 2. Extruder output is denoted by arrow Q, and the back flow ofthe extrudable material from the grooves 2 over the flights 3 in theaxial direction, i.e. downward in FIG. 1, by arrow Q₂.

FIG. 2 is a cross-sectional view of a part of the rotor according toFIG. 1 seen in the axial direction and from below. The numbering in FIG.2 corresponds to that of FIG. 1. For the sake of clarity, the rotor 1and an outer stator 4 disposed outside the rotor are shown as having astraight surface between them, although they are naturally circular inthe direction from which they are seen. The flight 3 is bevelled so thatclearance h₁ between the rotor 1 and the stator 4 at the front edge ofthe flight 3 in the direction of rotation of the rotor 1 is bigger thanclearance h₂ at the rear edge of the flight 3. The width of the flight 3in the circumferential speed v direction is denoted by s.

FIG. 3 shows pressure functions of the flight 3 between the rotor 1grooves 2. The numbering in FIG. 3 corresponds to that of FIGS. 1 and 2.Pressure functions P₁ to P₄ represent pressure functions at differentpoints of the flight 3, the pressure function P₁ standing for a pressurefunction at a lower point of the rotor in FIG. 1 than pressure functionsP₂ to P₄, and pressure function P₄ standing for a pressure function at ahigher point than pressure functions P₁ to P₃, pressure functions P₂ andP₃ being naturally between them. The maximum value of a pressurefunction is directly proportional to the viscosity η of the extrudablematerial, the circumferential speed v of the rotor 1, and the width s ofthe flight, and inversely proportional to the size of the clearance h₁,h₂ between the rotor 1 and the stator 4. The maximum value P_(max) of apressure function may be presented by means of the formula:

P _(max) =η×v×s÷(h ₁ +h ₂)².

Since the extrudable material is fed into the wider end of the rotor 1,its viscosity η is at its highest in the lower portion in FIG. 1,diminishing as the material melts and softens as it moves towards theupper end of the rotor 1. Likewise, the circumferential speed v of therotor 1 is naturally at its highest at the widest point of the rotor.Consequently, the pressure function is greater at the wider part of therotor 1 than at the narrower part. This means that the back flow Q₂advancing in the axial direction is always subjected to a pressurefunction that is greater than the current pressure function, whereby theback flow Q₂ is reduced. Surprisingly, this brings about the advantagethat although the flow Q₁ in the peripheral direction is relativelygreat due to the bevelled shape of the flight 3, the back flow Q₂,however, is not very great. Consequently extruder output Q remains good.The rotor 1 is designed to be such that the pressure according to FIG. 3is generated both outside and inside the rotor. This rising and fallingpressure provides a hydrodynamic bearing between the rotor and thestator. The variation in pressure also alternating increases anddecreases the material flow rate. By the action of the pressures on thedifferent sides of the rotor 1, the rotor 1 reaches an equilibrium suchthat the rotor always moves further away from the stator on the side ofwhich the pressure is higher and vice versa. In this way the clearancebetween the rotor 1 and the stators may adjust automatically. Thepressure function receives its energy from the circumferential speed vrotating the rotor 1.

FIG. 4 is a schematic cross-sectional side view of another extruderaccording to the invention. The numbering of FIG. 4 corresponds to thatof FIGS. 1 to 3. Outside the conical rotor 1 there is a conical outerstator 4 and inside it a likewise conical inner stator 5. The conceptconical as used in this application implicates that the shape of thedevice is conical at least as regards the feed and melt zones. The endportion of the device may be e.g. cylindrical or of the shape of anexpanding cone. In this case both sides of the rotor comprise an annularconical feed gap 9, disposed one within the other. The flight 3 isformed such that, as seen in FIG. 4, the clearance between its upperedge and the stator is bigger than the clearance between the lower edgeof the flight 3 in the Figure. The travel direction of the extrudablematerial in FIG. 4 is from below upwards. The rotor 1 comprises holes 6through which at least part of the extrudable material can flow from oneside of the rotor 1 to the other side, from one feed gap 9 to another.

FIG. 5 is a cross-sectional view of the rotor of FIG. 4 in the directionof its grooves. The numbering of FIG. 5 corresponds to that of FIGS. 1to 4. The grooves 2 are arranged to become alternately shallower anddeeper in the flow direction of the extrudable material. As a result,when the extrudable material moves from the bigger clearance h₁ to thesmaller clearance h₂, it is subjected to a greater pressure. The higherpressure on the extrudable material is partially released as the groove2 becomes deeper, and partly as the extrudable material moves throughthe hole 6 to the other side of the rotor 1. This way the abovedescribed advantageous bearing effect is brought about by the action ofa higher and a lower pressure. Because of the hydrodynamic bearingeffect, essentially no other bearing application is needed when theextruder is used. Should any other bearing application be used, it ispreferably arranged at the widest part of the rotor, and thus thesupporting effect of such bearing application will be optimal and therewill be no welt line. Furthermore, because of the holes 6, the possiblydifferent pressures on different sides of the rotor 1 can even out,whereby no great unilateral force can affect the rotor. In FIG. 5 theflow paths of the extrudable material are illustrated by arrows. Theholes 6 may be conical, i.e. converge in the direction of the materialflow. This way the material flowing through the holes 6 is subjected toorientation in the converging through-holes 6.

FIG. 6 is a schematic side view of a part of the rotor of a secondextruder according to the invention. The numbering of FIG. 6 correspondsto that of FIGS. 1 to 5. The grooves 2 are shaped such that the depth ofa groove changes in the transverse direction so that the deepest pointof the groove is close to the upper edge of the groove 2 in FIG. 6. Theflight 3 again is arranged to be narrow and sharp.

FIG. 7 shows a cross section of the rotor part of to FIG. 6 seen in theaxial direction from below. The numbering of FIG. 7 corresponds to thatof FIGS. 1 to 6. FIG. 7 clearly shows the shape of the groove 2, i.e.the front edge of the groove 2 is more steeply bevelled than the rearedge. FIG. 7 further shows the narrow and sharp shape of the flight 3.

FIG. 8 is a schematic side view of the rotor of a fourth deviceaccording to the invention. The numbering of FIG. 8 corresponds to thatof FIGS. 1 to 7. The lower part of the rotor 1 comprises normal grooves2 for extruding extrudable material upward in the Figure. From thegrooves 2 the extrudable material is transported to a ring groove 7 andfrom the ring groove 7 to a first auxiliary groove 2 a. From the firstauxiliary groove 2 a the material passes over an intermediate flight 3 ato a second auxiliary groove 2 b and from there further via dischargegrooves 2 c out of the extruder. The rotor 1 further comprises guideflights 3 b that are higher than the intermediate flights 3 a, wherebyno extrudable material is essentially transported over the guide flights3 b. Instead, essentially all extrudable material is led over theintermediate flight 3 a, whereby the extrudable material is subjected toa pressing pressure at the intermediate flight 3 a, the effect of saidpressure reducing as the extrudable material passes to the secondauxiliary groove 2 b. Consequently, the hydrodynamic bearing is obtainedby means of the intermediate flight 3 a. As regards the balancing of thebearing and the rotor, the first auxiliary groove 2 a, and the secondauxiliary groove 2 b, and the intermediate flight 3 a between them neednot extend along the length of the entire rotor 1. For the sake ofclarity, FIG. 8 only shows some of the grooves 2, 2 a to 2 c and flights3, 3 a to 3 c.

FIG. 9 is a schematic cross-sectional view of a detail of the rotorshown in FIG. 8 seen in the axial direction. The numbering of FIG. 9corresponds to that of FIGS. 1 to 8. For the sake of clarity, the rotor1 and the stator 4 are shown in FIG. 9 as having a straight surfacebetween them. Essentially all extrudable material 8 is led from thefirst auxiliary groove 2 a to the second auxiliary groove 2 b via theintermediate flight 3 a. The clearance h₁ of the intermediate flight 3 aat the front edge on the side of the first auxiliary groove 2 a isbigger than the clearance h₂ between the edge on the side of the secondauxiliary groove 2 b and the stator. The extrudable material 8 may bemainly e.g. polyethylene 8 a and additionally contain e.g. crosslinkedpolyethylene which will become crosslinked in the extruder. While in thegroove 2 a, the crosslinked polyethylene 8 a is still in a sphericalform, but when subjected to pressure at the intermediate flight 3 a, itorientates to an elliptical form, as illustrated in FIG. 7. In thismanner a product containing orienting and crosslinked polyethylenefibers is obtained. This kind of a product is very strong and durable.The extrudable product may be e.g. a plastic tube, a cable sheath, afilm or the like. The device according to FIGS. 6 and 7 produces aproduct whose crosslinked polyethylene fibers 8 a are essentially alloriented to an elliptical form. By the device according to FIGS. 1 to 5,at least some of the fibers can be oriented as the flow cross-sectionalsurface converges into elliptic form. Liquid crystalline plastic, forexample, is also a potential extrudable material, and will be defibratedby the stretching effect of the device. Preferably about 5 to 40% ofliquid crystalline plastic is mixed with matrix plastics. It isparticularly advantageous to use plastic mixtures in which the primarycomponent to be defibrated, such as LCP, is more fluid with respect toviscosity than the matrix plastics. The fibrillation of said componentis here intensified as the component is more fluid and hence tends toflow over the flight and is thereby stretched. A similar effect ispartly achieved when MD or LD polyethylene, into which a crosslinkedcomponent, such as peroxide, has been absorbed, is added to e.g. HDPEplastics. In this case the more flowing ingredient will be fibrillatedand simultaneously crosslinked by the action of heat. If thecrosslinking agent, such as peroxide, is selected such that it has anextremely accurate reaction temperature, it is possible that only thesubstance passing over the flight is crosslinked, as the temperature ofthe process is highest at that point. The device of the inventionproduces a product in which at least some of the fibers are oriented inthe direction of the helix. Glass fibers can also be used as reinforcingfibers.

The drawings and the associated description are intended only toillustrate the invention. The invention may vary in its details withinthe scope of the claims. If desired, the grooves and flights may thus bearranged in the stators instead of or in addition to the rotor 1. It isalso possible to use more than one rotor and two stators. Furthermore,it is possible to use e.g. only one stator and one rotor outside andinside it, whereby e.g. in the case of FIG. 4, reference number 1 wouldrefer to the stator, and reference numbers 4 and 5 to the rotors. It isessential to the invention, however, that there are at least two annularconical feed gaps 9.

What is claimed is:
 1. An extruder comprising rotor means and statormeans for defining at least two annular conical feed gaps for receivingextrudable material and for extrusion of the material from the extruder,said rotor means comprising at least one rotatable rotor, said statormeans comprising at least one stator, said at least one rotor or said atleast one stator being disposed between the at least two annular feedgaps, at least one of the rotor or stator means comprising means forconveying at least part of the extrudable material received in the atleast two feed gaps from a first part of the extruder to a second partof the extruder with the at least one rotor rotating at acircumferential speed and for subjecting the conveyed materialalternately to an increase and decrease in pressure so as to promote abalancing of the at least one rotor or the at least one stator betweenthe feed gaps, said means for conveying and subjecting comprising aplurality of grooves through which at least part of the conveyedmaterial passes from a first of said plurality of grooves to a second ofsaid plurality of grooves.
 2. An extruder according to claim 1, whereinsaid plurality of grooves are primarily helical in shape.
 3. An extruderaccording to claim 2, wherein said means for conveying and subjectingcomprises at least one flight disposed between said first and secondgrooves such that at least part of the material conveyed from said firstgroove to said second groove passes over said at least one flight, saidat least one flight having a front edge and a rear edge and beingdisposed on said at least one rotor with the front edge of the flightcloser to the at least one stator than the rear edge of the flight orbeing disposed on the at least one stator with the front edge of theflight closer to the at least one rotor than the rear edge of theflight.
 4. An extruder according to claim 2, wherein said plurality ofgrooves comprise holes through which at least part of the extrudablematerial can pass from one of the at least two feed gaps to another. 5.An extruder according to claim 4, wherein at least one of the pluralityof grooves alternately increases and decreases in depth along alongitudinal direction thereof.
 6. An extruder according to claim 2,wherein said conveying and subjecting means comprises at least oneflight disposed between said plurality of grooves, and wherein the atleast one flight is bevelled, said bevelled flight having a front edgeand a rear edge, the at least one rotor and the at least one statorbeing closer to each other at the rear edge of the bevelled flight thanat the front edge of the bevelled flight.
 7. An extruder according toclaim 2, wherein said means for conveying and subjecting comprises afirst auxiliary groove, a second auxiliary groove and an intermediateflight therebetween, said first auxiliary groove, second auxiliarygroove and intermediate flight being disposed in the extruder such thatessentially all of the extrudable material extruded from the extruderflows over the intermediate flight from the first auxiliary groove tothe second auxiliary groove.
 8. A method for extruding extrudablematerial comprising (a) providing an extruder comprising rotor means andstator means for defining at least two annular conical feed gaps forreceiving the extrudable material and for extrusion of the material fromthe extruder, said rotor means comprising at least one rotor, saidstator means comprising at least one stator, at least one of said rotoror stator means comprising means for conveying the extrudable materialreceived in the at least two feed gaps from a first part of the extruderto a second part of the extruder with the rotor means rotating at acircumferential speed and for subjecting the conveyed materialalternately to an increase and decrease in pressure so as to promote abalancing of the at least one rotor or the at least one stator betweenthe at least two feed gaps, said means for conveying and subjectingcomprising a plurality of grooves through which at least part of theconveyed material passes from a first of said plurality of grooves to asecond of said plurality of grooves; and (b) supplying the extrudablematerial to the extruder with the rotor means rotating at thecircumferential speed so as to cause the extrudable material to beconveyed from the first groove to the second groove, to be subjected tothe increase and decrease in pressure and to be extruded from theextruder.
 9. A method according to claim 8, wherein said plurality ofgrooves are primarily helical in shape.
 10. A method according to claim8, wherein said means for conveying and subjecting comprises at leastone flight disposed between said first and second grooves such that atleast part of the material conveyed from said first groove to saidsecond groove passes over said at least one flight, said at least oneflight having a front edge and a rear edge and being disposed on said atleast one rotor with the front edge of the flight closer to the at leastone stator than the rear edge of the flight or being disposed on the atleast one stator with the front edge of the flight closer to the atleast one rotor than the rear edge of the flight.
 11. A method accordingto claim 8, wherein said plurality of grooves comprise holes throughwhich at least part of the extrudable material passes from one of the atleast two feed gaps to another.
 12. A method according to claim 8wherein at least one of the plurality of grooves alternately increasesand decreases in depth along a longitudinal direction thereof.
 13. Amethod according to claim 8, wherein said conveying and subjecting meanscomprises at least one flight disposed between the plurality of thegrooves, and wherein the at least one flight is bevelled, said bevelledflight having a front edge and a rear edge, the at least one rotor andthe at least one stator being closer to each other at the rear edge ofthe bevelled flight than at the front edge of the bevelled flight.
 14. Amethod according to claim 8, wherein said means for conveying andsubjecting comprises a first auxiliary groove, a second auxiliary grooveand an intermediate flight therebetween, said first auxiliary groove,second auxiliary groove and intermediate flight being disposed in theextruder such that essentially all of the extrudable material flows overthe intermediate flight from the first auxiliary groove to the secondauxiliary groove before being extruded from the extruder.
 15. A methodaccording to claim 8, wherein the extrudable material supplied to theextruder is orientable and said means for conveying and subjectingcauses the extrudable material to become oriented in the extruder.
 16. Amethod according to claim 15, wherein the extrudable material is aliquid crystalline polymer.
 17. A method according to claim 8, whereinthe extrudable material comprises crosslinked polyethylene and the meansfor conveying and subjecting causes the extrudable material to become atleast partly oriented in the extruder.
 18. A method according to claim8, wherein the extrudable material comprises crosslinked polyethyleneand the means for conveying and subjecting causes the extrudablematerial to become essentially entirely oriented in the extruder.
 19. Amethod according to claim 8, wherein the extrudable material comprises amixture of a liquid crystalline plastic and a matrix plastic and themeans for conveying and subjecting causes the extrudable material to beextruded as a product comprising fibrous nets in the matrix plastic.